Every time (once, lol) I've replaced the batteries so far, it's gone into the 'I'm sorry, Dave, I don't recognize those fobs' state. Which is why the official stance is to have the SC replace the batteries. Yes, I know the batteries don't last a year (sometimes). So, doing this for a backup may not be the best failsafe course of action, just sayin'.Just carry the key without battery, and follow the "dead battery" procedure when you need to use the key as backup. Then it'll be equivalent to the Model 3's key card, although more bulky.
Now, of course, yes, the fobs can be used in the same mode as the 3, where (without the battery) you put them at the bottom of the door for the X, and near the wipers for the S (if I'm remembering correctly) and then at the base of the temperature sensor at the console to drive. I guess that's sorta like the card.....